The present invention relates to a plasma jet spark plug for an internal combustion engine which generates plasma to ignite an air-fuel mixture and to an ignition system for the plasma jet spark plug.
Conventionally, when an internal combustion engine such as automobile engine runs at low load (hereinafter referred to as “low load operation”), such as while starting or during idling, accidental firing due to unstable combustion tends to occur. In response, lowering the mixture ratio of air and fuel (hereinafter referred to as “the A/F ratio”) is performed to facilitate smooth ignition and prevent stalling. However, such an adjustment causes excessive fuel consumption. Therefore, improvement in the ignition characteristics of a spark plug, which achieves secure ignition and a stable combustion of the air-fuel mixture despite a high A/F ratio has been demanded.
A plasma jet spark plug is known as a spark plug with high ignitability as disclosed in Laid Open Japanese Patent Application Publication No. S56-98570. As used herein, “ignitability” refers to the ability of a spark plug or plasma jet spark plug to ignite the air-fuel mixture in the cylinder of an internal combustion engine. Such a plasma jet spark plug (igniter plug) includes a small electric discharge space and a circumferential face of a spark discharge gap between a center electrode and a ground electrode which is surrounded by an insulating material such as ceramic. High voltage is applied between the center electrode and the ground electrode in order to generate a spark discharge. The dielectric breakdown caused by the spark discharge causes a current flow at relatively low voltage. Further, the spark discharge transits and generates plasma in the spark discharge space to ignite the air-fuel mixture by supplying energy.
Plasma has a high ignitability and provides stable combustion at low load operation. However, plasma tends to cause an increase in temperature of a spark plug due to its high energy, thereby resulting in a significant wearing of the electrode of the spark plug. Japanese Patent Publication No. S56-98570 also discloses that plasma is generated to ignite the air-fuel mixture at low load operation. On the contrary, only the spark discharge is performed at the time of high load operation (hereinafter referred to as “high load operation”), such as at high speed running of an internal combustion engine, to prevent wearing out of the electrode as well as to improve the ignitability.
However, since a plasma jet spark plug according to the above-noted Japanese patent application has a construction in which a spark discharge gap is surrounded by a face made of an insulating material, a spark discharge ignites an air-fuel mixture, which is included in the spark discharge gap, at high load operation where only an ignition by the spark discharge is performed. Thus, poor ignitability and slow combustion may occur because a flame core cannot be formed in a flow of the air-fuel mixture in a combustion chamber.
The present invention is accomplished in view of the foregoing problems of the prior art and an object of the present invention is to provide a plasma jet spark plug which can improve the ignitability and durability thereof by forming a part of a spark discharge gap in the outside of the electric discharge space which generates plasma. An ignition system for the plasma jet spark plug is also provided.